Pressure vessel innovation, especially in the petroleum refining industry, is driven by the factors of utility, safety, reliability, costs and ease of operation and maintenance. This is especially true in the petroleum refining process of delayed coking in which large pressure vessels are employed to recover valuable products by thermally cracking heavy residual hydrocarbons. Heavy residual hydrocarbon, or resid, is the recovered bottom stream from the initial refining of crude oil or other oil sources such as shale oil, coal oil, or Fischer Tropsch synthetic oil.
Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit, and then pumping the heated heavy feed into a large steel pressure vessel commonly known as a coke drum. The unvaporized portion of the heated heavy feed settles out in the coke drum where the combined effect of retention time and temperature causes the formation of coke. Vapors from the top of the coke drum, which typically consist of steam, gas, naphtha and gas oils, are returned to the base of the fractionation unit for further processing into desired light hydrocarbon products. The operating conditions of delayed coking can be quite severe. Normal operating pressures in coke vessels typically range from 25 to about 50 pounds per square inch and the heavy feed inlet temperature may vary between 800° F. and 1000° F.
Coke vessels are typically large, cylindrical vessels commonly 19 to 30 feet in diameter and two to three times as tall having a top head and a funnel shaped bottom portion fitted with a bottom head and are usually present in pairs so that they can be operated alternately. Coke settles out and accumulates in the vessel until it is filled to a safe margin, at which time the heated feed is switched to the empty “sister” coke vessel. Thus, while one coke vessel is being filled with heated residual oil, the other vessel is being cooled and purged of hundreds to thousands of tons of coke formed in the vessel during the previous recovery cycle.
Removal of coke from a full coker vessel, also known as decoking, typically is a time consuming and potentially dangerous process that generally involves cooling the multi-ton coke mass with water, drilling and cutting the coke mass from the drum with a specialized drilling system and dumping the hot, disaggregated mass along with steam and hot water into a chute through a hole in the coke vessel bottom. Opening the hole in the coker vessel bottom (or the top hole for drill insertion) for coke removal in older systems involves removal of a head device, which is designed to tightly seal the coker vessel during the coking phase of the cycle. The process of removing and replacing the removable top head and bottom units of the vessel cover is called heading and unheading or deheading. It is dangerous work, with several risks associated with the procedures. There have been fatalities and many serious injuries. There is significant safety risk from exposure to steam, hot water, fires and repetitive stress associated with the manual unbolting work. Accordingly, the industry has devoted substantial time and investment in developing semi-automatic or fully automatic unheading systems, with attention focused on bottom unheading where the greatest safety hazard is present.
There are two commonly used methods to move the bottom head out of the way of the falling coke. The first is to completely remove the head from the vessel, perhaps carrying it away from the vessel on a cart. The other way of “removing” the bottom head is to swing it out of the way, as on a hinge or pivot, while the head is still coupled to the vessel. These systems all use a manual or semi-automatic bolting system that must be uncoupled with every decoking cycle and require that a pressure tight and leak free seal is re-established before the coking cycle can begin. Several coker vessel systems of the above described types are disclosed in: U.S. Pat. No. 6,264,829 (discloses a swing away hydraulically operated drumhead adapted for low headroom situations); U.S. Pat. No. 6,254,733 (depicting in the drawings a hydraulically removable drumhead); U.S. Pat. Nos. 6,066,237 and 5,876,568 (disclosing an apparatus for semi-automatically clamping and unclamping a drum bottom head); U.S. Pat. No. 5,947,674 (a drum head device removed by vertically oriented hydraulic cylinders adapted for lowering the head unit and moving it laterally aside); U.S. Pat. No. 5,785,843 (claims a process involving a swing away hydraulically operated drumhead adapted for low headroom situations); U.S. Pat. No. 5,581,864 (a remotely operated carriage mounted drumhead removal system); U.S. Pat. No. 5,500,094 (car mounted drumhead removal system that is horizontally movable); U.S. Pat. No. 5,228,825 (a device and method for deheading a drum comprising, in part, a cradle that holds the drum head for removal); U.S. Pat. No. 5,221,019 (a remotely operated cart removal system); U.S. Pat. No. 5,098,524 (a pivotally attached unheading device associated with clamps); U.S. Pat. No. 4,726,109 (a platform device lowers the drumhead and moves it laterally away).
All the above described bottom head removal systems pipe the heated feed into the coke vessel from the bottom through the center of the bottom head. Reorienting the bottom feed to the side above the unheading devices would eliminate many of the time consuming and unsafe tasks associated with unheading coker vessels and such systems are known in the older art. However, side entry use has been discontinued in coker vessels built and put into operation in the last 20 to 30 or more years because of significant problems maintaining the integrity of the seals between the head devices and the vessel resulting in significant leakage events and maintenance downtime. It is well known in the art that side entry feed systems result in differential thermal and weight loads at the flanged interfaces between the head devices and the vessels. These conditions create significant challenges for seal maintenance, thus there is a preference in the art for bottom entry feed systems, which makes decoking safety and efficiency improvements difficult. Recently, however, significant improvements in the process of opening and closing pressure vessels, such as coker vessels have been achieved; for example, the “unheading” valve described in PCT Patent WO 02/07371. This new valve easily and automatically opens and closes a coker drum and is repetitively operable through numerous coking/decoking cycles, thus eliminating the cyclic heading and deheading process as described above. However, to be repetitively and continuously operable through numerous coking/decoking cycles without removal, this type of valve closure requires a feed entry system laterally placed above the valve apparatus. Such a system is disclosed in U.S. patent application Ser. No. 10/043,527 which teaches a closed system that eliminates worker exposure during coker vessel decoking operations and increases coking capacity by reducing the coking cycle time. In one preferred embodiment that is particularly useful for retrofitting existing coker systems, a bottom adapter or transition piece, herein termed a spool, is interposed between the vessel bottom and the valve closure unit and pressure-tightly sealed to both. In this system, the side entry feed is most readily accomplished by means of a feed pipeline laterally attached to the adapting spool member. The spool member comprises a single, cylindrical unit with annular flanged surfaces on both ends for attachment between the coker vessel and the valve apparatus. However, even with improvements in flange and seal design over older systems, maintaining seal integrity at the spool/vessel and spool/valve interfaces continues to be a significant problem as a result of the differential thermal and weight loads attendant to the side entry feed configuration. Such differential loads result from asymmetrical coke accumulation and distribution on the lower portion of the coker drum, which causes high flange loads and high temperatures to be concentrated leading to flange stud yielding, chronic flange leaks and ultimately metal fatigue. Further exacerbating the problem are delayed coking process operating temperatures that range from ambient to about 1000° F., which causes uneven expansion and contraction of the spool and vessel flange diameters by as much as ⅛th inch every coking/decoking cycle. Such differential expansion between the drum and spool flange causes gasket failure. The present invention, directed to an insulated transition spool apparatus, solves these problems.